1. Field of the Invention
This invention relates to the field of semiconductor testing circuits.
2. Prior Art
A well known phenomenon associated with the life of electronic components is known as "infant mortality", a term which is used to describe the relatively high failure rate observed in new components. The initial high failure rate of components normally drops to a low value after a short period of operation, and continues low for a relatively long period of time. In order to avoid the effects of infant mortality in power semiconductor devices, it is common to "burn in" such components until the period of infant mortality is past. The "burn in" typically comprises alternately passing a high forward current through the device, and applying a reverse bias voltage approximately equal to the rated peak inverse voltage. For convenience, this is usually done at power line frequency, forward current being passed through the device on one-half of the cycle, and the rated peak inverse voltage being applied on the other half cycle.
The prior art method of accomplishing the burn-in involves connecting many of the devices being processed in parallel, each with a series resistor, so as to avoid variations in current due to differences in voltage drop among the devices being burned in, and also to avoid the difficulties which would accompany a device failure. The power dissipated in this series resistor is substantial and, in fact, the cost of this power represents a significant factor with respect to the total cost of the device.
Circuits have been devised by previous inventors whereby several devices under test (being "burned in") are put in series, and only one series resistor for the entire series string is required. Unfortunately, many semiconductor devices have very high peak inverse voltage ratings, for example 1500 volts, so that if more than a few are put in series, the inverse voltage required becomes excessive. Thus, this improvement, while reducing the per unit power cost somewhat, offers only a limited improvement.